Home, office and many wide-format ink jet printers use scanning printer head technology (scanning ink jet printers). Scanning ink jet printers tend to have relatively limited print speeds. As a result scanning ink jet printers are less well suited to high throughput commercial applications. In scanning ink jet printers the printer head moves (scans) across the substrate, whilst the substrate is stationary. As the printer head scans across the substrate surface (often across the width of the substrate) droplets of ink are ejected towards the substrate thereby forming an image. Often, to obtain the best possible resolution or highest print density the scanning head will scan over the same substrate region several times to overlay ink droplets. The result of the scanning action is the formation of a relatively thin printed strip on the substrate. Only after the scanning head has formed the desired printed strip is the substrate moved (typically along the length of the substrate) by the printer mechanism such that another strip can be printed. In this manner a scanning ink jet printer slowly prints the required image.
Very recently single pass ink jet printers have been developed. Single pass ink jet printers offer much higher print speeds and were designed with the requirements of high throughput commercial applications in mind. In single pass ink jet printing one or more inkjet printer heads are in a fixed position and the substrate is printed typically in a single pass as it travels under the printer head(s). In single pass ink jet printers the printer heads are much larger typically as wide as the substrate itself.
Single pass ink jet printing imposes even more stringent requirements on the ink jet printing ink and the colorant when compared to scanning ink jet printing. In scanning ink jet printers the ink jet printer heads can be cleaned many times during the printing of a single sheet of the substrate. In contrast, in single pass printers the heads cannot be cleaned nearly so frequently. In addition, the higher print speeds and large volumes of ink ejected per minute means the ink has to work extremely robustly with the single pass printer mechanism. Accordingly, ink jet printing inks and colorants for single pass ink jet printers should desirably provide even higher levels of reliable operation in the printer.
More specifically, the ink jet printer inks and colorants for single pass ink jet printers should desirably provide:    i) long open times (good latency) such that the ink components do not dry out on the printer nozzle even when the nozzle is inactive for extended periods of time;    ii) high stability such that none of the components of the ink tend to flocculate, aggregate, precipitate or otherwise produce oversized particulate matter;    iii) high optical density (even in a single pass);    iv) excellent droplet formation and droplet breakup characteristics even at very short timescales or high droplet ejection frequencies;    v) high reliability when the ink is ejected through nozzles over extended periods;    vi) relatively fast drying of the ink once printed onto the substrate;    vii) good toxicology and using the lowest possible amounts of volatile organic liquids;    viii) low foaming characteristics;    ix) good light fastness and image permanence;    x) excellent ink storage stability;    xi) sharp prints on highly porous media such as corrugate.
Single pass ink jet printers often utilise very different substrates from home or office printers. A home printer might well use expensive substrates such as high quality photo papers. Photo papers typically have a high whiteness, high gloss and a highly controlled microporous structure adapted specifically for controlled spreading and penetration of the ink jet printing ink. For single pass ink jet printing the substrates tend to be less expensive and their coatings, if any, tend to be less well adapted to receiving ink jet printing inks. For substrates such as these the previously known ink jet printing inks often tend to penetrate too deeply into the substrate cause a loss in optical density, wicking and/or strike through.
Pigment-based inks comprise pigment particles dispersed in the ink whereas dye-based inks comprise a dye dissolved in the ink. Pigment-based inks tend to have some advantages in certain applications. For example, the light and ozone-fastness of pigment-based inks tends to be superior to dye-based inks.
That said, if the ink is pigment-based rather than dye-based then meeting many of the above requirements can be even more difficult. For example it is known that in pigment-based inks the pigment particles may tend to agglomerate or flocculate over time thereby creating oversized particles which can block or impair the printing of the tiny ink jet printer nozzles. Also, it is known that pigment-based inks often tend to flocculate if the ink contains higher relative amounts of organic liquids or strongly adsorbing surfactants. Thus, in many respects dye rather than pigment-based inks are more suited to the high printer reliability requirements of single pass printing.
In our studies we have seen that it is extremely difficult to simultaneously satisfy many or all of the above requirements. Some requirements which are especially difficult to simultaneously achieve are high optical density, high colloidal stability of the pigment-based ink and high printer reliability.